Don't let themes change disabled config node background color

[bonanitech]

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

While testing the changes from #3730, I noticed that I left disabled config nodes out of #3564.

This PR fixes that by using the same background color as in the default theme.

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the forum/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality

[coveralls]

Coverage remained the same at 68.328% when pulling 744375b on bonanitech:disabled-config-node-background into 7072265 on node-red:master.

[Steve-Mcl]

Looks fine